1. Field of the Invention
This invention relates to polymer electrolyte membrane (PEM) fuel cells and methods for producing components thereof. More particularly, this invention relates to the design and construction of modular unit fuel cells that lower production costs.
2. Description of Related Art
A polymer electrolyte membrane (PEM) fuel cell is an electrochemical device comprising an anode electrode, a cathode electrode and an electrolyte in the form of a thin polymer membrane disposed between the anode electrode and the cathode electrode. Individual polymer electrolyte membrane fuel cells or fuel cell units are stacked with bipolar separator plates separating the anode electrode of one fuel cell unit from the cathode electrode of an adjacent fuel cell unit to produce polymer electrolyte membrane fuel cell stacks.
Electrochemical cells comprising polymer electrolyte membranes may be operated as fuel cells wherein a fuel and an oxidizer are electrochemically converted at the cell electrodes to produce electrical power, or as electrolyzers wherein an external electrical current is passed between the cell electrodes, typically through water, resulting in generation of hydrogen and oxygen at the respective electrodes of the cells. Fuel cells are energy conversion devices that use hydrogen and oxygen, usually from the air, to create electricity through a chemical conversion process, without combustion and without harmful emissions. The voltage and current output depends on the number of cells in the stack, total active surface area and efficiency. The basic process, for a single cell, is shown in FIG. 1.
Traditional fuel cell stacks 10, see FIGS. 2, 3 and 4 are made of many individual cells 20 which are stacked together. Such fuel cells typically have a bipolar separator plate (BSP) 12 made of machined graphite, a membrane electrode assembly (MEA) 14, gaskets 16, 18, a fuel manifold 24, and may have oxidizer and a coolant manifolds. See FIGS. 3 and 4.
For the proper operation of fuel cells, the hydrogen gas must be sealed inside the cell and separated from the gaseous oxidant (air or oxygen). In some fuel cells, cooling is required because of the heat generated during normal operation. This heat is commonly removed from the fuel cell stacks by liquid cooling, commonly using water as a coolant.
Additionally, it is critical that the BSP 12 be in intimate, continuous electrical contact with the MEA 14.
As shown in FIGS. 2 & 4, fuel cell stacks have typically used a “filter-press” structure, where thick and heavy “end plates” 32, 34 are placed at the ends of each fuel cell stack 10 and are held together by heavy tie-rods, or bolts 38 and nuts 40, or other fasteners.
The “filter press” structure is an attempt to serve two purposes: (i) sealing the hydrogen, the oxidant, and the liquid coolant, if used; and (ii) maintaining intimate electrical contact between the BSPs 12 and the MEAs 14, see FIGS. 2 and 4. Disassembly and analysis of fuel cell stacks built by traditional methods reveals that the “filter press” arrangement performs neither function very well. Such analysis has revealed evidence of incomplete electrical contact between BSPs 12 and MEAs 14, resulting in poor electrical conduction and lower cell performance. The analysis has also shown evidence of gas and liquid leakage.